1. Field of the Invention
The present invention relates, in general, to cooling tubes and is particularly, but not exclusively, applicable to cooling tubes used in a plastic injection-molding machine to cool plastic parts, such as plastic parisons or preforms. More particularly, the present invention relates to a structural configuration of these cooling tubes, and also to method of manufacturing and using such tubes, for example in the context of a manufacturing process for preforms made from polyethylenetetraphthlate (PET) or the like.
2. Related Art
In order to accelerate cycle time, molding machines have evolved to include post mold cooling systems that operate simultaneously with the injection molding cycle. More specifically, while one injection cycle is taking place, the post mold cooling system, typically acting in a complementary fashion with a robotic part removal device, is operative on an earlier formed set of molded articles that have been removed from the mold at a point where they are still relatively hot, but sufficiently solid to allow limited handling.
Post mold temperature conditioning (or cooling) molds, nests or tubes are well known in the art. Typically, such devices are made from aluminum or other materials having good thermal conductivity properties.
To improve cooling efficiency and cycle time performance, EP patent 0644 describes a multi-position take-out plate that has a capacity to store multiple sets of preforms for more than one injection cycle. In other words, each set of preforms is subjected to an increased period of accentuated conduction cooling by retaining the preforms in the cooling tubes for more than one injection cycle. With increased cooling, the quality of the preforms is enhanced. At an appropriate point in time, a set of preforms is ejected (usually by a mechanical ejection mechanism) from the take-out plate onto a conveyor to allow a new set of preforms to be inserted into the now vacant set of cooling tubes. EP patent 0644 is incorporated herein by reference.
In many other cooling tube arrangements, the preform (at some point, if not from the point of introduction) looses contact with the internal side walls of the cooling tube, which loss of thermal contact lessens cooling efficiency and causes uneven cooling. As will be understood, uneven cooling can induce part defects, including deformation of overall shape and crystallization of the plastic (resulting in areas that are visibly hazed). Furthermore, lack of contact can cause ovality across the circumference of the preform, while the loss of the cooling effect can mean that a preform is removed from the cooling tube at an excessively high temperature. In addition to causing surface scratching and overall dimensional deformation, premature removal of a preform at an overly high temperature can also result in the semi-molten exterior of preform sticking either to the tube or another preform; all these effects are clearly undesirable and result in part rejection and increased costs to the manufacturer.
European patent EP 0 266 804 describes an intimate fit cooling tube that is held within an end-of-arm-tool (EOAT) of a robot. The intimate fit cooling tube is water cooled and is arranged to receive a preform shortly after it has attained the glass-transition temperature that allows handling of its form without catastrophic deformation. More particularly, after the preform has undergone some cooling within the closed mold, the mold is opened, the EOAT extended between the cavity and core sides of the mold and the preform off-loaded from a core into the cooling tube that then acts to cool the exterior of the preform by a conduction process. However, as the preform cools it will shrink and therefore may loose contact across its entire circumference with the cooling tube yielding an uneven cooling effect.
U.S. Pat. Nos. 4,102,626 and 4,729,732 are further typical of prior art systems in that they show a cooling tube formed with an external cooling channel machined in the outer surface of the tube body, a sleeve is then assembled to the body to enclose the channel and provide an enclosed sealed path for the liquid coolant to circulate around the body.
WO 97/39874 discloses a tempering mold that has circular cooling channels included within its body.
EP 0 700 770 discloses another configuration that includes an inner and outer tube assembly to form cooling channels therebetween.
U.S. Pat. No. 4,208,177 discloses an injection mold cavity containing a porous element that communicates with a cooling fluid passageway subjecting the cooling fluid to different pressures to vary the flow of fluid through the porous plug.
U.S. Pat. No. 4,047,873 discloses an injection blow mold in which the cavity has a sintered porous sidewall that permits a vacuum to draw the parison into contact with the cooling tube sidewall.
U.S. Pat. No. 4,295,811 and U.S. Pat. No. 4,304,542 disclose an injection blow core having a porous metal wall portion.
A xe2x80x9cPlastics Technology Onlinexe2x80x9darticle entitled xe2x80x9cPorous Moldsxe2x80x9d Big Drawxe2x80x9d, by Mikell Knights, printed from the Internet on Jul. 27, 2002, discloses a porous tooling composite called METAPOR(trademark). The article discloses the technique of polishing this material to close slightly the pores to improve the surface finish and reduce the porosity.
An article from International Mold Steel, Inc., entitled xe2x80x9cPorous Aluminum Mold Materialsxe2x80x9d, by Scott W. Hopkins, printed from the Internet on Jul. 27, 2002, also discloses porous aluminum mold materials. The materials and applications disclosed in the above two articles refer to vacuum thermoforming of plastics in the mold itself, in which preheated sheets of plastic are drawn into a single mold half via a vacuum drawn through the porous structure of the mold half.
According to a first aspect of the present invention, structure and/or steps are provided for a tube assembly for operating on a malleable molded plastic part. The tube assembly comprising a porous tube having a profiled inside surface, and a vacuum structure configured to cooperate with the porous tube to provide, in use, a reduced pressure adjacent the inside surface. The reduced pressure causes an outside surface of the malleable molded plastic part, locatable within the tube assembly, to contact the inside surface of the porous insert so as to allow a substantial portion of the outside surface of the malleable part, upon cooling, to attain a profile substantially corresponding to the profile of the inside surface. In an embodiment of the invention, the porous tube is cylindrically-shaped, and the vacuum structure is provided by locating the porous tube in a tube body and by providing at least one vacuum channel adjacent the outside surface of the porous tube, in use, for connection to a vacuum source.
The inside surface of the porous tube having an internal profile that is substantially (if not highly and accurately toleranced to) the final dimensions of the molded part, the porous tube of the various embodiments of the present invention effectively causes, under cooling, a re-shaping of the molded part to its exact final shape defined by the profile of the insert. Indeed, the reduced pressure/effective vacuum acting through the porous material essentially acts to draw the malleable preform into the final shape whilst ensuring that cooling is optimized by continuous surface contact with a thermally efficient heat dissipation material and path.
According to a second aspect of the present invention, injection molding machine structure and/or steps are provided with a molding structure that molds at least one plastic part. Furthermore, at least one porous cooling cavity is configured to hold and cool the at least one plastic part after it has been molded by the molding structure. At least one vacuum channel is respectively configured to provide a lower-than-ambient pressure to the at least one porous cavity to cause the at least one plastic part to contact the inside surface of the at least one porous cavity.
According to a third aspect of the present invention, a method for shaping a malleable molded plastic part including the steps of: (i) receiving the molded plastic part into a porous tube; (ii) providing a reduced pressure adjacent a profiled inside surface of the porous tube causing a substantial portion of an outside surface of the molded plastic part to move into contact therewith and thereby attain a substantially corresponding shape; and (iii) extracting heat from the molded plastic part through a heat dissipation path to solidify the molded plastic part at least to the extent required to ensure that the shape of the outside surface of the molded plastic part is preserved; and (iv) ejecting the molded plastic article; wherein the outer surface of the molded plastic part is provided with a final shape that is defined by the profiled inside surface profile of the porous tube.
According to a fourth aspect of the present invention, structure and/or steps are provided for a tube assembly for operating on a malleable molded plastic part. The tube assembly comprising a tube body, and a porous insert located in the tube body. The porous insert includes an inside surface and an outside surface, the inside surface profiled to reflect at least a portion of the profile of the molded plastic part. The tube assembly further includes at least one vacuum channel in fluid communication with the porous insert. The vacuum channel configured for connection, in use, with a vacuum source to provide a reduced pressure adjacent the inside surface to cause an outside surface of the malleable molded plastic part, locatable within the tube assembly, to contact the inside surface so as to allow a substantial portion of the outside surface of the malleable part, upon cooling, to attain a profile substantially corresponding to the profile of the inside surface. The tube assembly also includes a cooling structure configured for connection, in use, with a heat dissipation path for cooling the molded plastic part in contact with the inside surface of the porous insert. Preferably, the porous insert has porosity in the range of about 3-20 microns. A cooling fluid passageway is disposed in the tube body adjacent the porous insert and is configured to carry a cooling fluid to extract heat from the porous insert.
According to another aspect of the present invention, structure and/or steps are provided for a tube assembly. The tube assembly comprising a tube with an inside surface provided on a porous substrate, and a fluid flow structure. The fluid flow structure is configured to cooperate with the porous substrate to cause, in use, a malleable molded plastic part, locatable within the tube assembly, to be drawn into contact with the inside surface so as to allow a substantial portion of an outside surface of the malleable part, upon cooling, to attain an outside profile substantially corresponding to the profile of the inside surface.
According to an embodiment of the invention, the porous substrate includes an inside surface and an outside surface, the inside surface profiled to reflect at least a portion of the profile of the molded plastic part; and a vacuum channel located adjacent the outer surface, the vacuum channel supporting, in use, an initial establishment of a differential pressure from the outside surface of the porous substrate to the inside surface thereof, to induce contact, in use, between the received molded plastic part and the inside surface.
According to yet another aspect of the present invention, structure and/or steps are provided for an end-of-arm tool. The end-of-arm tool comprising a carrier plate for mounting, in use, to a robot in a molding system, and at least one tube assembly arranged on the carrier plate. The tube assembly is configured for receiving, in use, a molded plastic part. The tube assembly comprising a porous tube having an inside surface and an outside surface, the inside surface profiled to reflect at least a portion of the profile of the molded plastic part, and a vacuum structure. The vacuum structure is configured to cooperate with the porous tube to provide, in use, a reduced pressure adjacent the inside surface to cause an outside surface of a malleable molded plastic part, locatable within the tube assembly, to contact the inside surface of the porous insert so as to allow a substantial portion of the outside surface of the malleable part, upon cooling, to attain a profile substantially corresponding to the profile of the inside surface.
The present invention advantageously provides a cooling tube structure that functions to cool rapidly and efficiently a just-molded plastic part located within the cooling tube, thereby improving robustness of the preform and generally enhancing cycle time. Moreover, in the context of cooling PET and the unwanted production of acid aldehyde arising from prolonged exposure of the preform to relatively high temperatures, the rapid cooling afforded by the present invention beneficially reduces the risk of the presence of unacceptably high levels of acid aldehyde in the finished plastic product, such as a drink container. Beneficially, the present invention seeks to maintain a required and defined shape of the molded part, such as a preform.